The present invention relates to a process for the production of polyester polyols having low viscosity values, prepolymers prepared from these polyester polyols, polyurethanes prepared from these polyester polyols and/or from these prepolymers, and to processes for their production.
The use of ε-caprolactone as a constituent of polyester polyols, which can in turn be used, for example, in the production of polyurethanes, has been known for a long time. See W. Meckel, W. Goyert and W. Wieder in Thermoplastic Elastomers, A Comprehensive Review, N. R. Legge, G. Holden and H. E. Schroeder, Hanser Publishers Munich 1987, p. 17).
The synthesis of poly-ε-caprolactones from monomeric ε-caprolactones is also known. According to the disclosure of U.S. Pat. No. 2,933,477, the production of poly-ε-caprolactones takes place by reacting bifunctional starter molecules with lactones. The molecular weight of the poly-ε-caprolactone obtained by this process is substantially determined by the molar ratio of starter molecules and monomeric ε-caprolactone molecules used. According to U.S. Pat. No. 2,933,477, starter molecules are those originating from the group R′(ZH)2, in which Z substantially denotes —O—, —NH—, or —NR″— and in which R′ denotes a hydrocarbon residue which is selected from the group consisting of alkylene, arylene, aralkylene and cycloalkylene. Examples of these starters are, for example, diols, such as ethylene glycol, amino alcohols, such as ethanolamine, or diamines, such as piperazine, etc. Hydroxyl group-terminated polyethers such as, for example, polypropylene oxides, polytetrahydrofurans etc., can also be used as starter molecules. The polymerization takes place in a temperature range of 50 to 300° C., preferably 120 to 200° C., preferably in the presence of a catalyst, which can be basic, acidic or neutral or can be a transesterification catalyst.
Compared with structurally similar polyadipates, poly-ε-caprolactones are distinguished by their increased hydrolytic stability when used as soft segment components in polyurethane systems. This is why they are used despite their increased costs. However, they have a tendency towards augmented crystallization, which means that, because of this hardening tendency, they are unsuitable for many applications. This problem can be remedied according to DE-A 1 946 873, by modifying poly-ε-caprolactones by incorporating dicarboxylic acids and diols. Polyurethane elastomers made from poly-ε-caprolactones modified in this way are also distinguished by increased hydrolytic stability with a reduced or non-existent tendency towards crystallization and hardening.
To produce the modified poly-ε-caprolactones, the ε-caprolactone, dicarboxylic acid and diol components are mixed and heated to temperatures of preferably 100 to 250° C., thus forming polyester polyols which have OH numbers of 40 to about 80 mg KOH/g, depending on the feed ratios, with water being eliminated. The proportion of ε-caprolactone can range from 10 to 75 wt. %, but preferably from 25 to about 70 wt. %. The typical representatives for preparing polyester polyols include, for example, succinic acid, adipic acid etc., which can be used as dicarboxylic acids, and ethylene glycol, butylene glycol, etc. which can be used as diols. In addition, small quantities of representatives with higher functionalities, such as glycerol, 1,1,1-trimethylolpropane, etc. may also be used.
From DE-A 2 115 072, the use of certain catalysts (e.g. antimony halides) and special ways of conducting reactions (e.g. pre-reaction of ε-caprolactone with polyols and subsequent reaction with polycarboxylic acid) are known for the production of the modified poly-ε-caprolactones (i.e. mixed polyester polyols).
These mixed polyester polyols are used in particularly high-value applications such as, for example, floppy disks (see U.S. Pat. No. 5,955,169), for the microencapsulation of water-soluble, water-dispersible or water-sensitive materials (see U.S. Pat. No. 5,911,923), in the production of ophthalmic lenses (see U.S. Pat. No. 5,880,171), bone substitute material for stabilizing damaged nasal bones (U.S. Pat. No. 5,810,749), production of spherical polyurethane particles (see EP-A 0 509 494), etc.
In addition, polyether esters containing caprolactone units, e.g. based on polyoxyethylene-polyoxypropylene-polyoxyethylene block copolymers, are interesting building blocks in medical applications. These are interesting since they contain hydrophilic structural elements (PEO) which give the system special, controllable properties in relation to gelling behavior in water, degradation kinetics and the temperature dependence of the rheological behaviour. (See D. Cohn et al., Biomaterials 27 (2006) 1718).
Where polyesters are used, e.g. in polyurethane (PU) applications, in addition to the properties of the PU end product, the production process thereof is also the subject of improvement efforts. It is of decisive importance, for example, that the processing properties of a reacting polyurethane melt must be easy to handle. This is particularly true of its viscosity, which, as far as possible, should not exceed a certain limit. Melts of too high a viscosity can be poured into molds only with difficulty, for example, or can no longer be completely mixed with the chain extender within a given time, and so products produced in this way are of no value. It is known that systems based on polyether polyols have lower viscosities, and so this problem can be solved in some cases by replacing the polyester polyol with a polyether polyol if the application allows.
It is also known that the viscosity of polyols generally rises with increasing molecular weight. (See, for example, P. C. Hiemenz: Polymer Chemistry, The Basic Concepts, Marcel Dekker, Inc. New York, p. 104, 1984). The above-mentioned processing problem therefore occurs particularly when a) polyester polyols, b) polyester polyols with higher molecular weights and c) polyester polyols with higher molecular weights and higher functionalities are used, with the processing problem becoming progressively more acute from a) to b) to c). Nevertheless, it may be necessary to implement even the particularly critical cases b) and c), with the disadvantages hitherto associated therewith, e.g. when comparatively flexible cast elastomers with comparatively high softening points have to be prepared. In this case, for example, a comparatively high proportion of a chain extender must then be reacted with a prepolymer having a comparatively large number of NCO groups in order to be able to achieve a high degree of hard segment oligomerization and, associated therewith, a high softening point. To reduce the hardness, it is necessary in such cases to use a polyol in which the chain is as long as possible and, depending on the requirements of the application, a polyester polyol that is as long-chained as possible. However, based on what was stated above, precisely this combination is particularly problematic.
One solution to this problem consists in using polyol-initiated poly-ε-caprolactones. Compared with polyadipates that are comparable in terms of molecular weight and functionality, poly-ε-caprolactones are distinguished by the fact that they exhibit considerably lower viscosity. Thus, from a technological point of view, they provide a thoroughly satisfactory solution to the problem described.
Unfortunately, however, compared with the polyadipates that are comparable in terms of molecular weight and functionality, poly-ε-caprolactones are significantly more expensive. Therefore, poly-ε-caprolactones cannot be used economically in many applications.
The object of the present invention was therefore to provide polyester polyols which, in comparison to similar, conventional polyester polyols, exhibit significantly lower viscosities and, in addition, which can be prepared by technically simple, economically favorable means, as well as a process for the production thereof.
In this context, conventional polyester polyols are those made up entirely or predominantly of polycarboxylic acids or derivatives thereof with at least 2 and no more than 6, preferably 4, carboxyl groups, and a total of 4 to 12 carbon atoms, i.e. for example adipic, glutaric, succinic, phthalic acid etc., and which are produced at temperatures of ≧180° C. with the elimination of water or of a low molecular-weight, usually monofunctional alcohol.
Surprisingly, it has been found that polyester polyols produced from particular starting compounds, which are reacted together in a particular sequence under defined conditions, exhibit significantly lower viscosities and the process for their production is technically simply and economical.